Just to follow up and close this thread, I found the dialog box belonged to Adobe Acrobat Reader DC and was set to be used by my Safari browser for the website (I don’t know why). I downloaded Reader from the Adobe website onto another machine, set it to open fresh pdfs on the website and got the same Cyrillic message. I don’t know what’s up with Adobe. Thank you all for indulging me during my exploration.

Cheers,
Darrell